Phase-change type optical information recording media have been known as information recording media on which information recording, erasing, rewriting, and reproduction are performed optically by using laser beams. Information recording, erasing, and rewriting are performed on a phase-change type optical information recording medium by utilizing a phenomenon in which a recording layer included in the medium is changed in phase reversibly between a crystalline phase and an amorphous phase. Generally, when recording information, a recording layer is allowed to melt by irradiation of a laser beam at a high power (recording power) and cooled rapidly, so that an irradiated portion is changed in phase to an amorphous phase, thereby allowing the information to be recorded. On the other hand, when erasing the information, the recording layer is raised in temperature by irradiation of a laser beam at a lower power (erasing power) than the power used for recording and cooled gradually, so that a laser beam irradiated portion is changed in phase to a crystalline phase, thereby allowing the recorded information to be erased. Thus, in a phase-change type optical information recording medium, a laser beam that is modulated in power level between a high power level and a low power level is irradiated to a recording layer, thereby making it possible to record new information while erasing information that has been recorded, i.e., to rewrite information (see, for example, “Basics and Applications of Optical Disk Storage” by Yoshito TSUNODA et al., The Institute of Electronics, Information and Communication Engineers, 1995, Chapter 2).
Further, phase-change type electrical information recording media also have been known on which information recording is performed by changing the state of a phase change material in a recording layer by, instead of irradiation of a laser beam, Joule heat generated by application of a current. In such a phase-change type electrical information recording medium, a phase change material in a recording layer is changed in state between a crystalline phase (low resistance) and an amorphous phase (high resistance) by Joule heat generated by application of a current, and a difference in electrical resistance between the crystalline phase and the amorphous phase is detected and read as information. When a current is applied to a recording layer of an amorphous phase sandwiched between electrodes gradually, the recording layer is changed in phase to a crystalline phase at a certain threshold current, so that the electrical resistance decreases rapidly. Further, by applying a large current pulse having a short time pulse width to the recording layer of a crystalline phase, the recording layer is allowed to melt and is cooled rapidly, thereby regaining the recording layer of an amorphous phase of high resistance. Thus, such recording media can be used as rewritable information recording media. The difference in electrical resistance between a crystalline phase and an amorphous phase can be detected easily by usual electrical means, and thus by using a recording layer that is changed in phase reversibly between a crystalline phase and an amorphous phase, a rewritable information recording medium can be obtained (see, for example, “Basics of Amorphous Semiconductor” by Makoto KIKUCHI, Ohmsha, Ltd., 1982, Chapter 8).
Examples of phase-change type optical information recording media include a 4.7 GB/DVD-RAM that has been commercialized by the inventors. The 4.7 GB/DVD-RAM, which is shown in FIG. 21 as an information recording medium 1000, has an information layer 1010 of an eight-layer structure in which, seen from an incident side of a laser beam 10, a first dielectric layer 1002, a second dielectric layer 1003, a crystalline nucleation layer 1004, a recording layer 1005, a third dielectric layer 1006, a fourth dielectric layer 1007, a light absorption correction layer 1008, and a reflective layer 1009 are laminated in this order. The information layer 1010 is laminated to a dummy substrate 1012 with an adhesive layer 1011.
The first dielectric layer 1002 and the fourth dielectric layer 1007 have an optical function and a thermal function. For the optical function, an optical distance is adjusted so that the light absorption efficiency with respect to the recording layer 1005 is increased, and thus a change in reflectance between a crystalline phase and an amorphous phase is increased, thereby allowing the signal amplitude to be increased. For the thermal function, the substrate 1001, the dummy substrate 1012, and the like that have low resistance to heat are insulated from the recording layer 1005 that is heated to a high temperature in recording. A mixture of 80 mol % ZnS and 20 mol % SiO2 (hereinafter, which may be referred to as “(ZnS)80(SiO2)20 (mol %)”), which is used generally for the first dielectric layer 1002 and the fourth dielectric layer 1007, is an excellent dielectric material that is transparent, has a high refractive index, a low thermal conductivity and a good thermal insulation property, and is excellent in mechanical properties and moisture resistance.
As a material of the crystalline nucleation layer 1004, Sn—Te or Pb—Te is used, which is a material that accelerates crystallization of the recording layer 1005 when it is provided in contact with the recording layer 1005. By using such a material, the crystallization ability of the recording layer is allowed to be increased without decreasing the signal amplitude of the information recording medium 1000 and with archival characteristics secured, and not only an initial recording/rewriting capability but also excellent archival characteristics (capability of reproducing recorded information that has been stored for a long term) and archival overwrite characteristics (capability of recording/rewriting information that has been stored for a long term) are realized (see, for example, JP 2001-273673 A).
As a material of the recording layer 1005, a pseudobinary phase change material represented by a point on a line of GeTe—Sb2Te3 is used, thereby realizing an excellent repeated rewriting capability.
The second dielectric layer 1003 and the third dielectric layer 1006 have a function of preventing mass transfer from being caused between the first dielectric layer 1002 and the recording layer 1005 and between the fourth dielectric layer 1007 and the recording layer 1005. The mass transfer is a phenomenon caused when (ZnS)80(SiO2)20 (mol %) is used for each of the first dielectric layer 1002 and the fourth dielectric layer 1007. In this phenomenon, when recording/rewriting is performed repeatedly by irradiation of laser beams to the recording layer 1005, S is diffused in the recording layer 1005. When S is diffused in the recording layer 1005, the repeated rewriting capability is deteriorated (see, for example, N. Yamada et al., Japanese Journal of Applied Physics, Vol. 37 (1998), pp. 2104-2110). In order to prevent the deterioration in repeated rewriting capability, a nitride containing Ge should be used for the second dielectric layer 1003 and the third dielectric layer 1006 (see, for example, WO 97-34298 and JP 10 (1998)-275360).
The above-mentioned technology has allowed an excellent rewriting capability and high reliability to be achieved and led to the commercialization of the 4.7 GB/DVD-RAM.
Furthermore, various forms of technologies for increasing the capacity of information recording media further have been studied. For example, for optical information recording media, a technology is studied for achieving high-density recording using a laser beam with a reduced spot diameter, which can be obtained by the use of a blue-violet laser beam having a wavelength shorter than that of a conventionally used red laser beam, or the use of a substrate that is thinner on an incident side of a laser beam in combination with the use of an objective lens having a high numerical aperture (NA). Furthermore, studies also have been made on a technology in which a recording capacity is doubled by using an optical information recording medium including two information layers and with respect to these two information layers, recording/reproduction is performed using a laser beam incident from a side of one side of the optical information recording medium (see, for example, JP 2000-36130 A).
In order to perform recording using a laser beam with a reduced spot diameter to increase the capacity of information recording media, it is necessary to use an optical information recording medium that allows even a recording mark reduced in size to be formed into an excellent shape. When recording is performed using a laser beam with a reduced spot diameter, the laser beam is irradiated to a recording layer for a relatively shorter time. Thus, to form a recording mark reduced in size, it is necessary to use a material that realizes a high crystallization speed for the recording layer, or to provide a film having a high effect of accelerating crystallization in contact with the recording layer.
In the optical information recording medium including two information layers (hereinafter, which may be referred to as a “two-layer optical information recording medium) that allows information recording/reproduction to be performed with respect to the two information layers by irradiation of a laser beam from a side of one side, the laser beam that has been transmitted through an information layer (hereinafter, referred to as a “first information layer”) arranged at a position closer to an incident surface of the laser beam is used to perform recording/reproduction with respect to an information layer (hereinafter, referred to as a “second information layer”) arranged at a position farther from the laser beam incidence surface. To this end, it is required that a much thinner film be used as a recording layer in the first information layer so that a higher light transmittance can be obtained. However, when the recording layer becomes thinner, crystalline nuclei formed when the recording layer is crystallized are reduced, and further a distance in which atoms are allowed to move becomes shorter. For theses reasons, a thinner recording film hinders the formation of a crystalline phase more (which means the crystallization speed is decreased).
Further, when an information transfer rate is increased by decreasing a time for information recording on an information recording medium, a time for crystallization is reduced. Thus, to realize information recording media adaptable to a high transfer rate, it also is required that the recording layer have a higher crystallization ability. Further, when information recording is performed at a high transfer rate, the recording layer that has been raised in temperature is cooled at a speed higher than that achieved when recording is performed at a lower transfer rate, and thus microcrystalline nuclei are less likely to be formed in an amorphous phase after recording. In other words, a more stable amorphous phase is likely to be obtained. An amorphous phase tends to be changed to a still more stable energy state after long-term storage. Therefore, when information recording is performed at a high transfer rate, it becomes more difficult to allow the recording layer to be crystallized, resulting in a deterioration in archival overwrite characteristics.
According to the test performed by the inventors, it has been revealed that by the use of a recording layer having a composition in which Sn is substituted for a part of Ge in a pseudobinary composition represented by a point on or near the line of GeTe—Sb2Te3, the crystallization speed of a recording layer can be increased. However, in this case, as the amount of Sn as a substituent is increased, the change in optical property between a crystalline phase and an amorphous phase becomes small, thereby decreasing the signal amplitude. Further, when the amount of Sn is increased, a recorded amorphous phase is crystallized gradually, and thus the archival characteristics are deteriorated in the case where recording is performed at a particularly low transfer rate.
As described above, as the capacity of information recording media increases, it becomes difficult to realize in a single information recording medium both the archival overwrite characteristics at a high transfer rate and the archival characteristics at a low transfer rate.
To allow the crystallization ability of the recording layer to be increased without decreasing the signal amplitude and with the archival characteristics secured, as described above, a crystalline nucleation layer made of Sn—Te or the like that accelerates crystallization of the recording layer should be provided in contact with the recording layer. According to the test performed by the inventors, it has been revealed that the crystallization ability of the recording layer can be increased by the use of Bi—Te for the crystalline nucleation layer. However, Sn—Te and Bi—Te have a low melting point, and thus a phenomenon is caused in which when the recording layer is raised in temperature in recording, Sn—Te or Bi—Te melts and, after repeated rewriting operations, is mixed into the recording layer gradually. As a result, the repeated rewriting capability is deteriorated.
As described above, in conventional information recording media, it is difficult to realize both an excellent information erasing rate (a phase change to a crystalline phase) as well as excellent archival overwrite characteristics at a high transfer rate and excellent archival characteristics at a low transfer rate, and at the same time, an excellent repeated rewriting capability.